Since the organic EL devices are self-luminescent devices, they are bright, excellent in visibility, and capable of giving clear display as compared to liquid crystal devices, and studies thereon have actively been conducted.
In 1987, C. W. Tang et al. of Eastman Kodak Co. have turned the organic EL device using an organic material into practical utilization by developing a multilayer device wherein various functions are respectively distributed to materials. They stack a fluorescent material capable of transporting electrons and an organic substance capable of transporting holes, and injected both of the charges into the fluorescent material layer to emit a light, thereby achieving a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (see Patent Document 1 and Patent Document 2, for example).
Patent Document 1: JP-A-8-48656
Patent Document 2: Japanese Patent No. 3194657
From differences in process of device preparation and characteristics of materials, the organic EL devices are classified into devices of vapor deposition type using a low molecular material and devices of wet-process type mainly using a polymer material.
The vapor deposition type device requires a vacuum vapor deposition apparatus for film formation. Contrary, in the case of the wet-process type device, film formation is easily carried out by coating a substrate with a coating liquid and then removing a solvent in the coating liquid. Therefore, the wet-process type device is obtainable by a simple production process and at a low production cost. Since the coating is easily performed by an ink jet method and a printing method, no expensive equipment is required for the production.
Polymer materials such as poly(1,4-phenylenevinylene) (hereinafter abbreviated as PPV) have generally been used as materials for the production of the wet-process type device (see Non-Patent Document 1, for example).
Non-Patent Document 1: Applied Physics Letters, 71-1, page 34, (1997)
Also, an organic EL device having a hole injection layer, a hole transport layer, and an electron transport layer in addition to an luminescent layer, thereby further segmentalizing various roles has been studied. Poly(ethylenedioxythiophene)/poly(styrenesulfonate) (hereinafter abbreviated as PEDOT/PSS) has widely been used as a hole injection or transport material for forming a hole injection layer or a hole transport layer by coating (see Non-Patent Document 2, for example).
Non-Patent Document 2: Optical Materials 9 (1998) 125
However, a coating liquid of PEDOT/PSS is an acidic aqueous solution since the coating liquid is an aqueous gel dispersion hydrated by PSS to which a molecular chain of PEDOT gives an ionic interaction. Therefore, the coating liquid has difficulties in use such as corrosion by the coating liquid of a coating/printing device such as an inkjet ejection head.
Also, it has been pointed out that PSS in the coating film adversely affects on an anode and that water used for the coating liquid remains in the device to result in deterioration during driving. Further, a thiophene ring in PEDOT is said to be reduced due to electron influx. Having these difficulties, PEDOT/PSS cannot be considered as a satisfactory hole injection/transport material, and satisfactory device characteristics were not obtained, particularly, in terms of durability.
As hole injection/transport materials other than PEDOT/PSS, polymers such as poly(N-vinylcarbazole) (hereinafter abbreviated as PVK) have been known (see Non-Patent Document 3, for example).
Non-Patent Document 3: Journal of Organic Molecular Electronics and Bioelectronics of Japan Society of Applied Physics, Vol. 11, No. 1, pages 13 to 19, (2000)
In the polymer materials including both PEDOT/PSS and PVK, there is a concern for influences caused by a low molecular material used for polymerization and crosslinking of the polymer. Also, since the polymer material in general is a mixture of various molecular species, the composition thereof is not perfectly uniform, thereby making it difficult to equalize the performance of devices produced.
In order to solve the above problems, Japanese Patent Application Nos. 2004-089836 and 2004-090334 propose an arylamine compound of a single molecular species and excellent in amorphousness and derivatives thereof, and also propose highly efficient and highly durable organic EL devices using a coated film layer of the these compounds.
Since a work function exhibited by these compounds is close to that of ITO, the compounds are suitably used as the hole injection material like PEDOT/PSS. However, in the case where the compounds are used singly as the hole injection/transport material, efficiency of an organic EL device was lower than that obtained by using PVK since the work function of the compound is low as the hole transport material.
A high performance amorphous material that is of a single molecular species and singly usable as a hole injection/transport material has not been obtained. Therefore, a highly efficient and highly durable organic EL device has not been obtained.
Meanwhile, Japanese Application No. 8-49045 (publication no. JP-A -8-259940) has proposed an organic EL device using a compound having a structure similar to that of this invention. The invention proposes a production of an organic EL device having a high heat stability by using an arylamine compound characterized by having 3 or more anilinobenzene molecular structures bonded to a single molecule or by using a derivative thereof. However, since it is difficult to prepare a coating liquid from the compound having such structure due to its characteristic of being hardly soluble to organic solvents, a device production method thereof is a vapor deposition method.